Spatial and rotational quality assurance of 6DOF patient tracking systems

Abstract

Purpose: External tracking systems used for patient positioning and motion monitoring during radiotherapy are now capable of detecting both translations and rotations. In this work, the authors develop a novel technique to evaluate the 6 degree of freedom 6(DOF) (translations and rotations) performance of external motion tracking systems. The authors apply this methodology to an infrared marker tracking system and two 3D optical surface mapping systems in a common tumor 6DOF workspace. Methods: An in-house designed and built 6DOF parallel kinematics robotic motion phantom was used to perform motions with sub-millimeter and subdegree accuracy in a 6DOF workspace. An infrared marker tracking system was first used to validate a calibration algorithm which associates the motion phantom coordinate frame to the camera frame. The 6DOF positions of the mobile robotic system in this space were then tracked and recorded independently by an optical surface tracking system after a cranial phantom was rigidly fixed to the moveable platform of the robotic stage. The calibration methodology was first employed, followed by a comprehensive 6DOF trajectory evaluation, which spanned a full range of positions and orientations in a 20 × 20 × 16 mm and 5• × 5 • × 5• workspace. The intended input motions were compared to the calibrated 6DOF measured points. Results:The technique found the accuracy of the infrared (IR) marker tracking system to have maximal root-mean square error (RMSE) values of 0.18, 0.25, 0.07 mm, 0.05• , 0.05 • , and 0.09• in left-right (LR), superior-inferior (SI), anterior-posterior (AP), pitch, roll, and yaw, respectively, comparing the intended 6DOF position and the measured position by the IR camera. Similarly, the 6DOF RSME discrepancy for the HD optical surface tracker yielded maximal values of 0.46, 0.60, 0.54 mm, 0.06• , 0.11 • , and 0.08 • in LR, SI, AP, pitch, roll, and yaw, respectively, over the same 6DOF evaluative workspace. An earlier generation 3D optical surface tracking unit was observed to have worse tracking capabilities than both the IR camera unit and the newer 3D surface tracking system with maximal RMSE of 0.69, 0.74, 0.47 mm, 0.28• , 0.19 • , and 0.18 • , in LR, SI, AP, pitch, roll, and yaw, respectively, in the same 6DOF evaluation space. Conclusions: The proposed technique was found to be effective at evaluating the performance of 6DOF patient tracking systems. All observed optical tracking systems were found to exhibit tracking capabilities at the sub-millimeter and subdegree level within a 6DOF workspace. C 2016 American Association of Physicists in Medicine. [http://dx

Towards frameless maskless SRS through real-time 6DoF robotic motion compensation

“…The proposed solution is to place the robotic system beneath the patient’s neck, away from the treatment beam, and control their cranial motion from that position. Since the calibration technique, and the motion compensation algorithm, are general approaches to associate the coordinate frames considered for motion correction and perform real-time correction, no fundamental changes to the overall control algorithm are expected (Belcher et al, 2016). Also important to note is the potential difference between the healthy volunteers used in this study, and patients who may be more difficult to stabilize in a clinical setting.…”

“…To this end, the tracked surrogates, such as the reflective IR markers used in this study, must be rigidly connected to the internal tracking target. Unfortunately, examination of the accuracies of various 6DOF tracking systems is beyond the goals of this study, which focuses on the robot and the motion control algorithm, however, some clinically suitable head tracking methods include the use of IR markers attached to a patient dental block, or a 3D surface map tracking of the rigid facial structures (Belcher et al, 2016; Cerviño et al, 2010; Jia et al, 2015; Schöffel et al, 2007; Wang et al, 2010; Wiersma et al, 2013). …”

“…In order to perform motion compensation and effectively cancel out the real-time measured 6DOF motion of the virtual point with respect to its initial position, a calibration procedure was developed to connect the three coordinate frames of the camera, the virtual point, and the parallel kinematics robot (Belcher et al, 2016; Kabsch, 1976). By knowing the displacement vectors and rotation matrices which can be used to associate these three coordinate frames, one can intelligently use the head motion measured using the camera and determine what trajectory for the robotic motion system would be necessary to effectively cancel out these 6DOF positioning errors.…”

Development of twist‐correction system for radiotherapy of head and neck cancer patients

“…For the systems excluding the 6DoF couch, Dhabaan et al reported that pitch and roll angles can be corrected by mounting the customized device on the top of the treatment couch in the cranial position . Belcher et al and Ostyn et al developed an electromechanical robotic system that can provide precise 6DoF motion trajectories . These systems can correct the rotational position of the patient by themselves; therefore, they might be able to correct the twist with the couch, which has the ability to achieve rotational correction.…”

A novel dynamic robotic moving phantom system for patient‐specific quality assurance in real‐time tumor‐tracking radiotherapy

“…14 robotic motion phantom using parallel links was proposed 15 and they subsequently evaluated its positioning accuracy. 16 A four-axis moving phantom for patient specific QA, in which four high precision prismatic actuators are used to control the center-of-gravity of the phantom, was also proposed recently. 17 The proposed moving phantom provides sufficiently high accuracy but can be costly.…”